The invention relates to a multi-stage compression refrigeration apparatus having a multiplicity of compression means for compressing a refrigerant in multi-stages.
A typical multi-stage compression refrigeration apparatus for use in a refrigerator and an air conditioner includes a rotary compressor consisting of a first and a second stage compression means which are housed in an enclosed container and each have a roller for compressing a refrigerant in the respective cylinder. The compressor performs compression of the refrigerant in two stages, first by the first stage compression means serving as a low-pressure compressor and then by the second stage compression means serving as a high-pressure compressor adapted to further compress the refrigerant gas compressed by the first stage low-pressure compressor.
Such a multi-stage compression refrigeration apparatus can attain a high compression ratio while suppressing variations of torque per one compression.
However, such multi-stage compressor has a drawback in that when a refrigerant has a high specific heat ratio, the second stage compression means has a low suction efficiency because it receives hot refrigerant heated by the first stage compression means. The multi-stage compressor also suffers from a further disadvantage that the temperature of the refrigerant is heated in the second stage high-pressure compression means to a great extent that the lubricant used therein will be thermally hydrolyzed into acids and alcohols, particularly when ester oil (for example, polyol ester, POE) is used. These acids disadvantageously develop sludges which tend to clog capillary tubes of the compressor, degrade the lubricant, and hence lower the performance of the apparatus.
In order to circumvent these problems, some compressors are provided with a cooling unit for cooling the refrigerant gas discharged from the first stage compression means before it is supplied to the second stage high-pressure compression means, thereby sufficiently lowering the temperature of the refrigerant gas discharged from the second stage compressor. For example, a known type of such multi-stage compression refrigeration apparatus as shown in FIG. 4 has: a multi-stage compressor 411 which consists of a first stage low-pressure compression means and a second stage high-pressure compression means; a condenser 412; a first decompression means 413, an intercooler 414, a second decompressor means 415, and an evaporator 416. The refrigerant exiting the condenser 412 is diverted into two parts, with one part passed to the intercooler 414 via the first decompression means 413, but the other part passed from the second decompression means 415 directly to the evaporator 416 so that the refrigerant flowing into the second decompression means 415 undergoes heat exchange with the intercooler 414. The refrigerant exiting the evaporator 416 is fed to the first stage compression means of the multi-stage compressor 411. On the other hand, the part of the refrigerant that has passed through the intercooler 414 is mixed with the refrigerant discharged from the first stage low-pressure compression means before entering the second stage compression means.
Thus, this multi-stage compression refrigeration apparatus has a refrigeration cycle as depicted in the P-h diagram (solid line) shown in FIG. 5. In this conventional apparatus, the enthalpy of the refrigerant is reduced by xcex4Ho, as shown in FIG. 5, by the heat exchange with the intercooler 414, so that the refrigerant is cooled before it flows into the second decompression means 415. Thus, this arrangement may increase an enthalpy difference across the evaporator 416.
However, in the conventional apparatus as mentioned above, the pressures of the refrigerant gas taken in the low-pressure and high-pressure compression means are almost the same (equilibrium pressure) during an early stage of startup. As a result, if the low-pressure compression means is larger in displacement volume than the high-pressure compression means, the amount, and hence the discharge pressure, of the refrigerant gas discharged from the former exceeds that of the latter compression means, thereby causing a backflow of the gas from the compressor to the intercooler 414.
The intercooler 414 is then heated by the backflow of refrigerant gas from the low-pressure compression means, which in turn results in a failure of adequate cooling of refrigerant fed to the second decompression means 415 by the intercooler 414. Hence, the apparatus disadvantageously takes time to attain supercooling to generate a large enthalpy difference xcex4Ho (shown in FIG. 5) that can be obtained under stable normal operation.
The invention is aimed to overcome these problems by providing an efficient multi-stage compression refrigeration apparatus which includes a first stage low-pressure compression means and a second stage high-pressure compression means. The apparatus comprises an intercooler for cooling the refrigerant gas discharged from the low-pressure compression means before it is fed to the high-pressure compression means, so that the refrigerant gas discharged from the high-pressure compression means has suppressed temperature. To shorten time for the apparatus to reach its stable operation following a startup, the apparatus is provided with a one-way valve to prevent backflow of the refrigerant gas from the first stage compression means to the intercooler.
In accordance with one embodiment of the invention, there is provided a multi-stage compression refrigeration apparatus including a compressor having a first stage low-pressure compression means and a second stage high-pressure compression means, a condenser, a first decompression means, a first intercooler, a second decompression means, and an evaporator. The refrigerant exiting the condenser is diverted into first and second parts, with the first part passed to the first intercooler via the first decompression means, while the second part is passed to the evaporator via the second decompression means and the first intercooler so that the refrigerant undergoes heat exchange with the first part in the first intercooler. The refrigerant exiting the evaporator is then fed to the first stage low-pressure compression means, and, when discharged from the first stage low-pressure compression means, mixed with the first part of the refrigerant exiting the first intercooler at a merging point upstream of the second stage high pressure compression means before the refrigerant is fed to the second stage compression means. The first stage low-pressure compression means has a larger displacement volume than the second stage high-pressure compression means. Between the first intercooler and the merging point, a one-way valve is provided to permit the refrigerant to flow only in the direction from the first intercooler to the merging point.
In this arrangement, the apparatus can: suppress sufficiently low the temperature of the refrigerant gas discharged from the second stage high pressure compression means; and prevent backflow of refrigerant from the first stage low pressure compression means to the first intercooler.
The apparatus may further comprise a second intercooler between the evaporator and the first stage low pressure compression means, which intercooler permits heat exchange between the first and the second parts of the refrigerant while passing through the second intercooler. This arrangement may create a larger enthalpy difference in the evaporator than conventional apparatuses during an early stage of startup.
The apparatus may have a third intercooler between the first intercooler and the merging point so that the refrigerant exiting the condenser undergoes heat exchange with the third intercooler, wherein the refrigerant exiting the third intercooler is passed through the one-way valve and fed to the second stage high pressure compression means together with the refrigerant discharged from the first stage low pressure compression means. This arrangement may advantageously enhance the above effects.
The refrigeration apparatus may further comprise a third decompression means for decompressing the second part of the diverted refrigerant after the refrigerant has undergone heat exchange with the second intercooler. The temperature of the refrigerant entering the evaporator is further lowered in this arrangement.